I think you're confusing several things here.
First of all, 'S30' is a series designation. The S30-series ( which includes several different models which wore the '240Z' variant name ) started in 1969 and went on until 1978. Different prefixes and suffixes to that 'S30' series helped to identify variants and sub variants ( like 'HS30-H', 'S30-S', 'HLS30-U' and 'PS30-SB' for example ) and even the 'S31' was technically an S30 series variant. 'C10' and 'C110' are series designations for the Skyline range.
When you see the letters and numbers 'S44', 'S45' and 'S46' in conjunction with the above series designations, what you are actually seeing are calendar year dates in the form of Japanese 'nengo'. This 'nengo' system links the formal name of each Emperor's reign with a number that corresponds to the years since he ascended the throne. The Emperor we knew as 'Hirohito' ascended the throne in 1925, so 1925 was the first year of his 'Showa' reign. The abbreviated 'S44', 'S45' etc are Showa years. Hence 'S44' is 'Showa 44', and this corresponds to 1969 on the Gregorian calendar ( 1925 + 44 = 1969 ).
So, you will often see an 'S30' or a 'C10' with the date 'S45' or 'S46' ( 1970 or 1971 ) next to it. Don't confuse factory model designations with 'nengo'. The 'nengo' is just being used to identify the production year.
'Showa' ended in 1989, and we are now in the 24th year of 'Heisei'. 2012 is 'H24'.
Probably not explained very well, but I hope it helps.